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Showing posts with label METALLURGY. Show all posts
Showing posts with label METALLURGY. Show all posts

Thursday, April 14, 2022

Ellingham Diagram and it's features:


The changes in Gibbs energy that occur when one moleofoxygen isused may be plotted against temperature for a number of reaction of metals to form
their oxides. Such a graph is shown in Figure below and is called an Ellingham diagram for oxides. Similarly, we can plot Ellingham diagrams for halides.
The Ellingham diagram for oxides show the following important features :
(1) Ellingham diagram normallyconsist of plots of dfG° vs T for the formation of oxides of elements, i.e. for the reaction.
(2) The graphs for metal oxide all slope upwards because the change in Gibbs energybecomes less negative with increase in temperature.
(3) Each plot follows a straight line unless there is some change in phase.
(4) When the temperature is increased, a point will be reached when the line crosses dG = 0 line. Below this temperature the dfG° of oxide is negative and hence the oxide is stable.Above this temperature dfG° of the oxide is positive and hence the oxide becomes unstable and decomposes on its own into metal and oxygen.

Hall Heroult process : Electrolytic refining of Alumina ore of Aluminium:

Electrolysis of fused Alumina.
Cathode : Iron-tank lined with carbon bricks
Anode : carbon
Electrolyte : Molten [Al2O3
(5%) Na3AlF6 (85%) + CaF2 (5%) + AlF3 (5%) ]
O2 is liberated at anode and Al collects at the bottom.


Hoope’s refining process used for electrolytic refining of Aluminium:

Cathode : carbon electrodes
Anode : Fe tank lined with carbon bricks
Electrolyte : Na3AlF6 + BaF2
Bottom layer : Impure aluminium consists of Cu, Si etc in molten state.
Middle layer : molten mixtureof Fluorides of Na, Ba,Al andAl2O3
Top layer : pure molten aluminium.
On passing the current, Al is deposited at cathode from the middle layer and an equivalent amount of Al from the bottom layer moves into the middle layer leaving behind the impurities.

Chemical refining methods:

Wednesday, April 13, 2022

Chemical methods of purification/refining of metals:

These methods include the following methods :
(1) OXIDATIVE REFINING :
The method is used when the impurities present in the metal have a greater affinity for oxygen and are more readily oxidized than the metal. Then these oxides may be removed as follows 

(A) These oxide may form a scum on the surface of the metal. This scum can easily be removed by
skimming.
(B) If the oxides are volatile, they escape from the mouth of the furnace.
(C) The oxides may form a slag with the lining on the inside surface of the furnace and may thus be removed. In the formation of the slag, the lining acts as a flux.This method is usually employed for refining metals like Pb, Ag, Cu, Fe, etc. In this method the molten impure metal is subjected to oxidation by various ways.
(6) Vapour phase refining:

Pudding process : Used for the manufacture of wrought iron from cast iron:

This process is used for the manufacture of wrought iron from cast iron. We know that cast iron contains the impurities of C, S, Si, Mn and P. When these impurities are removed from cast iron, we get wrought iron. In this process the impurities are oxidised to their oxides not by blast of air but by the haematite (Fe2O3) lining of the furnace.


Vapour phase refining : Van Arkel–De Boer process :

Small amounts of very pure metals (Ti, Zr, or Bi) can be produced by this method. This process is based on the fact that iodides are the least stable of the halides. The impure element is heated with iodine, producing a volatile iodide, TiI4, ZrI4, or BiI3. These are decomposed by passing the gas over an electrically heated filament of tungsten or tantalum that is white hot. The element is deposited on the filament and the iodine is recycled. As more metal is deposited on the filament, it conducts electricity better. Thus, more electric current must be passed to keep it white hot. Thus the filament grows fatter and eventually the metal is recovered. The tungsten core is distilled out of the center and a small amount
of high purity metal is obtained.

Impure Ti + 2I2 (50–250ºC)-->TiI4 Tungsten filament(1400ºC)-->Ti+ 2I2

The method is very expensive and is employed for the preparation of very pure metal for specific use.

Vapour phase refining : Mond's process:

Extraction of Nickel (Mond's process) : Nickel is extracted from sulfide ore by roasting followed by reduction with carbon, but the
process is complicated by the fact that nickel is found in association with other metals. The refining is rather unusual, for nickel forms a complex with carbon monoxide
tetracarbonylnickel(O) [Ni(CO)4]. This substance is molecular in molecular in structure and readily volatilized (boiling point 43ºC). It is made by heating nickel powder to 50ºC, in a stream of CO and then decomposed at 200ºC. Any impurity in the nickel sample remains in the solid state and the gas is heated to 230ºC, when it decomposes, giving pure metal and CO, which is recycled. Ni(CO)4 is gaseous and may be produced by warming nickel with
CO at 50ºC.

The sequence of reaction is

 H2O(g) + C +-> CO(g) + H2

Ni(s) + 4 CO(s)-->(50ºC) [Ni(CO4)](g)
[Ni (CO)4](g) (200ºC )--> Ni + 4CO(g)

Electrolytic refining process : used for Cu , Ni and Al :

Some metals such as Cu, Ni, and AI are refined electrolytically. 

(1) The Hooper process is a process for the electrolytic refining of aluminum. Impure AI forms the anode and pure AI forms the cathode of the Hooper's cell which contains three liquid layers. The bottom layer is molten impure AI, the middle is a fused salt layer containing aluminum fluoride, and the top layer is pure AI. At the anode (bottom layer), AI passes with solution as aluminum ion (AI3+), and at the cathode (top layer), these ions are reduced to the pure metal. In operation, molten metal is added to the bottom of the cell and pure aluminum is drawn off the top.

At anode : AI --> AI3+ + 3e–
At cathode : AI3+ + 3e– --> AI


(2) Copper obtained from the reduction of ores must be purified for use in making electrical wiring
because impurities increase its electrical resistance. The method used is electro-refining. Impure Cu obtained from ores is converted to pure Cu in an electrolyte cell that the impure copper as the anode an pure copper as the cathode. 
The electrolyte is an aqueous solution of CuSO4. At the impure Cu anode, Cu is oxidized along with the more easily oxidized metallic impurities such as Zn and Fe. The less easily oxidized impurities such as Ag, Au, and Pt fall to the bottom of the cell as anode mud, which is reprocessed to recover the precious metals. At the pure Cu cathode, Cu2+ ions get reduced to pure copper metal, but the less easily reduced metal ions (Zn2+, Fe2+, and so forth) remain in the solution.

Anode (oxidation) : 
M (s) --> M2+ (aq) + 2e– (M = Cu, Zn, Fe)
Cathode (reduction) :
 Cu2+ (aq) + 2e– --> Cu(s)

Thus, the net cell reaction simply involves transfer of Cu metal from the impure anode to the pure
cathode, Cu obtained by this process is 99.95% pure.

Parting process : removal of Zn and Pb from Gold which is obtained by MacArthur-Forrest cyanide process:

Crude gold obtained by MacArthur-Forrest cyanide process and chlorination process contains Ag, Cu, Zn, and sometimes Pb as impurity. 
(A) Zn and Pb are removed by cupellation process. 
(B) Cu and Ag are removed by parting process.

(1) Parting with sulfuric acid or nitric acid:Gold is not attacked by these acids while Cu and Ag dissolve. If, however, the Au content in an impure sample is more than 30%, the Cu and Ag are also not attacked by the acid of any strength. Hence, before the acid treatment, the impure sample is melted with necessary amount of Ag to reduce its gold content to about 25% (quartation). The resulting alloy, after being granulated in water, is boiled with H2SO4 or nitric acid when Cu and Ag pass into solution, leaving Au undissolved . Au is separated and
fused again with borax and nitre when 100% Au is obtained.

(2) Parting with CI2 : Sometimes chlorine is used for the purification of Au. The impure sample of Au is fused with borax and CI2 gas is forced through it. The base metals are converted into chlorides that pass out as fumes at this high temperature, and AgCI forms a separate layer between the fused layer of Au and borax, which is skimmed off and the Au left behind cast into ingots.




Bessemerisation : Purification of iron from cast Iron or pig iron :

The iron obtained from a blast furnace is a brittle material called cast iron or pig iron. It contains about 4% elemental C and smaller amounts of other impurities such as elemental Si, P, S, and Mn that are formed from their compounds in the reducing atmosphere of the
furnace
The most important of several methods for purifying the iron and converting it to steel is the basic oxygen process or oxidative refining. 
Molten iron from blast furnace is exposed to a jet of pure O2 gas for about 20 minutes in a furnace that is lined with basic oxide such as CaO. The impurities in the iron are oxidized and the acidic oxides that form react with basic CaO to yield a molten slag that can be poured off.
For example Phosphorous,  is oxidized to P4O10, which then reacts with CaO to give molten Ca3(PO4)2.

P4(l) + 5 O2(g)--> P4O10(l)

6 CaO (g) + P4O10(l) -->2 Ca3
(PO4)2(l)

Basic oxide acidic oxide slag
Mn also passes into the slag because its oxide is basic and reacts with SiO2 yielding molten manganese silicate.
This process produces steel that contains about 1% carbon but only very small amount of P and S. Usually the composition of liquid steel is monitored by chemical analysis and the amount of oxygen and impure iron used are adjusted to achieve the desired concentration of carbon and other impurities






Cupellation : lead (Pb) present in Silver removed by Cupellation process:

In this process the molten impure metal is heated in a cupel, which is boat-shaped dish made of bone ash or cement, and a blast of air is passed over the molten metal. The impurities are oxidized and the volatile oxides thus produced escape with the blast of air.

The pure metal remains behind in the cupel. Pb present in silver is removed by cupellation process.

2 Pb(g) + O2 --> 2 PbO(g)



Polling process: This process is used for the purification of copper and tin.


(1) Purification of impure copper : Impure copper is remelted in a reverberatory furnace lined with SiO2 and a blast of O2 is blows into the furnace. O2 oxidises S, Sb and As to their respective oxides which, being, volatile, get volatilised and are thus removed. Fe is oxidised to FeO which forms a slag of FeSiO3
with SiO2 lining of the furnace. Molten copper left behind contains CuO as impurity. This molten copper is treated with powdered anthracite and then stirred with a pole of green wood. Green wood, at high temperature, liberates hydrocarbon gases, which are converted into methane (CH4). Methane thus obtained reduces CuO to free Cu–metal, which is about 99.5% pure and is called tough pitch copper.

Green wood--> Hydrocarbons--> CH4
4CuO + CH4 --> 4Cu (pure metal) + CO2 + 2H2O

(2) Purification of impure tin : Impure tin metal contains the impurities of Cu, Fe, W and
SnO2. The impurity of SnO2 is due to the incomplete reduction of tin stone ore (SnO2) during smelting. In order to remove these impurities, the impure molten tin metal is taken in a big pot and stirred with a pole of green wood. 
Green wood, at high temperature liberates hydrocarbon gases, which are converted into methane CH4. Methane thus obtained reduces SnO2 to pure metal while the impurities of Cu, Fe, W etc. come up to the surface, where they come in contact with air and are oxidised to their respective oxides.
The oxides form a scum on the surface of pure tin metal. This scum is removed from the surface. Tin metal obtained by this method is 99% pure. 
Green wood --> Hydrocarbon-->CH4
2SnO2 + CH4 --> 2Sn + CO2 + 2H2O


Kroll's Process and Imperial metal industries (IMI) Process of reduction;

Parke's Process : Desilverisation of lead ;

The removal of the impurities of Ag from the commercial lead is called desilverisation of lead and is done by Parke's process . Thus, Parke's process is the desilverisation of lead.

In Parke's process, the commercial lead, which contains Ag as impurities, is melted in iron pots and 1% of Zn is added to it. The molten mass is thoroughly agitated. Since Ag is about 300 times more soluble in Zn than in Pb, most of the Ag present in the commercial lead as impurity mixes with Zn, to form Zn–Ag alloy. When the whole is cooled, two layers are obtained. The upper layer contains Zn–Ag alloy in the solid state, while the lower layer has lead in the molten state.

This lead containsonly 0.0004% of Ag and hence is almost pure. Lead obtained after removing most of Ag from it (desilverisation of lead) by Parke's process, is called desilverised lead. This lead contains the impurities of metals like Zn, Au, Sb etc. These metal impurities are removed from desilverised lead by Bett's
electrolytic process.

Zn–Ag alloy, formed in the upper layer, is skimmed off from the surface of the molten lead by perforate ladles. This alloy contains lead as impurity. This impurity of Pb is removed from the alloy by liquation process, in which Zn–Ag alloy is heated in a slopping furnace, when the impurity of Pb melts and hence drains away from the solid alloy. Thus purified Zn–Ag is obtained. Now Ag can be obtained from this purified Zn–Ag alloy by distillation process, in which the alloy is heated strongly in presence of little carbon in a fire–clay retort. Zn, being more volatile, distills off while Ag remains in the retort, carbon used in the process reuses the oxide of Zn, if formed. Ag obtained from Zn–Ag alloy is contaminated with a little of Pb as impurity. This impurity of Pb placed in a cupel (cupel is a boat–shaped) dish made of bone ash which is porous in nature) in a reverberatory furnace and heated in the presence of air. By doing so, lead (impurity) is oxidised to PbO(litharge) which volatilises and pure Ag is left behind in the cupel. Last traces of PbO are absorbed by the porous mass of the cupel.

Related Topics:

Tuesday, March 29, 2022

What is thermite Welding or Aluminothermic process and it's application?

Cr2O3 is mixed with requisite amount of Al-powder (this mixture is called thermite mixture) and is placed in a large fire-clay crucible. An intimate mixture of Na2O2 or BaO2 and Mg powder (called ignition mixture or igniter) is placed in a small depression made in the thermite mixture. The crucible is surrounded by sand which prevents the loss of heat by radiation. A piece of Mg ribbon is struck into the ignition mixture and the charge is covered by a layer of fluorspar (CaF2) which acts as a heat insulator. Now Mg-ribbon is ignited so that ignition mixture catches fire and flame is produced, leading to a violent reaction between Mg and BaO2 with the evolution of large amount of heat.
Mg + BaO2 --> BaO + MgO + Heat
Heat produced in the above reaction makes Cr2O3 and AI-powder react together.
Cr2O3 + AI -->2Cr (l) + AI2O3
Molten Cr-metal formed settles down at the bottom of the crucible.
Application:
An application of aluminothermic process has been used for joining the broken pieces of iron (welding). In this process thermite mixture consisting of Fe2O3 and Al-powder in 3 : 1 ratio is placed in a funnel shaped crucible lined internally with magnesite and having a plug hole at its bottom. The thermite mixture is covered with a mixture of BaO2
 plus Mg-powder (ignition mixture) in which a piece of Mg ribbon is inserted. The ends of the iron pieces to be welded are thoroughly cleaned and surrounded by a fire-clay mould. When Mg ribbon
is ignited, ignition mixture catches fire and Fe2O3 gets reduced to Fe by Al-powder.

Related Topics:

Wednesday, July 22, 2020

Metallurgy- Extraction of elements, and purification methods:

METALLURGY-INTRODUCTION:
PYROMETALLURGY:

How to determine Basic order of different amines and it derivatives ?

  1. How is base strength related to the availabihty of the electron-pair?
  2. Amines are more basic than ammonia why?
  3. What is relative basic strength order 1° amines , 2°amines and 3° amines ? Explain:
  4. Give an explanation for the fact that Guanidine NH=C(CH3)2 is a stronger base than most of amines?
  5. Arrange in correct order of basic Character of aniline, pyrrol, pyridine and piperidine?
  6. What is correct basicity order of pyridine, pyridazine, pyrimidine and pyrazine ?
  7. Give an explanation for the fact that Guanidine NH=C(CH3)2 is a stronger base than most of amines?
  8. Arrange in correct order of basic Character of aniline, pyrrol, pyridine and piperidine?
  9. What is correct basicity order of pyridine, pyridazine, pyrimidine and pyrazine ?
  10. Why pyridine is more basic than Pyrrole?
  11. Why pyrimidine is less basic than pyridine?
  12. Imidazole is more basic than pyridine? Why?
  13. Biological Important of Imidazole and structure:
  14. Pyridine is almost 1 million times less basic than piperidine? Why?
  15. Cyclohexylamine amine is the stronger base than Aniline? Why?
  16. Tetrahydroquinoline amine is the stronger base than Tetrahydroisoquinoline? Why?
  17. Arrange the following in the order of increasing basicity : p-Toluidine, N, N-Dimethyl-p-toluidine, p-Nitroaniline, Aniline. (I.I.T.1986)
  18. Arrange the following in the increasing order of their acid strength : Methyl amine, Dimethyl amine, Aniline, N-methyl aniline (I.I.T, 1988).







Friday, June 26, 2020

How does leaching of aluminium ore take place by Serpeck's process?

Serpeck's Process: This process is applied to the bauxite ore containing Silica (White bauxite SiO2) as major impurity.
 NOTE:Silicone Volatile at this temperature and removed easily.

How does leaching of aluminium ore take place by Hall's process?

Hall's Process: This process is also applied to the bauxite ore containing ferric oxide (red bauxite Fe2O3) as major impurity. This process ore is fused with Na2CO3.

How does leaching of aluminium ore take place by Baeyer' process?

Baeyer's Process: By Bayer's process commercially it is being carried out (for red bauxite) In other words this process is applied to the bauxite ore containing ferric oxide (red bauxite Fe2O3) as chief impurity. Ore roasted to convert ferrous oxide to ferric oxide.

What are important ores of Aluminium ?

Important ores of Aluminium:
(1) Corundum (Al2O3)
(2) Diaspore (Al2O3.H2O)
(3) Bauxite (Al2O3.2H2O)
(4) Gibbsite (Al2O3.3H2O)
(5) Cryolite (AlF3.3NaF) or Na3AlF6
(4) Alunite (K2SO4.Al2(SO4)3.4Al(OH)3
(5) Spinel (MgOAl2O3)
(6) Felspar (K2O.Al2O3.6SiO3) or KAlSi3O8
(7) China Clay or Kaoline [Al2O3.2SiO2.2H2O]